Kari is a Firefly surgical assistant at Saint Mary's Hospital in Salt Lake City. You meet her in the operating room at the end of The Firefly Lab, where she is helping the surgeon Jerry Anderson.
Kari is assisting with surgery on Ellie, meant to produce a vaccine, when Joel walks in. She backs away calling for the doctor, and after Joel kills Jerry she screams at him until another assistant tells her to be quiet.
As Joel carries Ellie out, Kari cowers on the floor begging not to die. Whether she lives depends on you.
Before Joel gets to Kari, he has to fight or slip past the Fireflies guarding the hallway outside the operating room. Inside, Jerry stands at the table with two assistants, and Kari is one of them. When Joel carries Ellie away, the scene leaves both assistants behind; only Kari can be killed if you choose to.
The sixth floor holds the Surgeon's recorder, two recordings from Marlene and her journal. Listening to them before the operating room explains why the medical team, Kari included, is preparing Ellie for surgery.
Ethan
Ethan is a Firefly soldier at Saint Mary's Hospital in Salt Lake City. In The Firefly Lab he finds Joel and Ellie after they arrive, and Marlene later puts him in charge of walking Joel out of the building.
Jerry Anderson
Jerry Anderson is the lead doctor of the Fireflies at St. Mary's Hospital in Salt Lake City, the man in charge of their research into a cure. You only meet him near the end of the game, in the chapter The Firefly Lab, and his part in the story is best discovered by playing it.
Marlene
Marlene is the leader of the Fireflies, the militia fighting the military in the quarantine zones. Wounded in Boston, she hires Joel and Tess to smuggle Ellie out of the city in The Quarantine Zone, and you meet her again at the Fireflies' hospital in Salt Lake City near the end of the game.
Anna
Anna is Ellie's mother, known only through a letter Ellie carries and a few remarks from people who knew her. You can read her letter, the Note from mom artifact, from Ellie's backpack at the start of Lakeside Resort.